An example of such a system is a cellular system employing a number of base stations (each serving a geographical area or "cell") with which mobile radios in the form of portable handsets can communicate by means of a radio link. Each base station emits a radio signal on a characteristic channel. When it is required to establish a radio link between a base station and a portable handset the handset undergoes a channel selection procedure with the object of identifying the particular base station producing the strongest valid signal at the handset, normally the nearest base station. The invention is concerned with this channel selection procedure which, when successfully completed, is followed by the establishment of a voice link between the portable handset and the selected base station.
In known cellular radio telephone systems, channel selection follows a procedure governed by an algorithm which results in the execution of the series of steps shown in FIG. 1 of the accompanying drawings. After initiation (step 1) the procedure requires that the portable handset tunes to each of the possible channels in turn (step 2). The handset checks the validity of each channel (step 3) by determining if particular data sequences are received. These particular messages are called the word sync (ws) message and the system parameter overhead message (spoh) which contains an auxiliary system identifier (ASID) number. If both signals are received, the channel number, signal strength and ASID number are stored (step 4) in a channel table. When all channels have been treated in this way (step 5), the channel having the strongest signal with a matching ASID number is selected (step 6). As the system parameter overhead messages transmitted by the base stations are normally sent once per second, it may take up to one second to process each channel and therefore completion of the whole procedure can take several seconds. Because the known method includes storage of data of all channels, there is a requirement for considerable memory for storage of channel numbers, ASID values and signal strengths. The invention aims to provide a channel selection means and method which are quicker than the conventional method and more economical in storage requirements.